The present invention relates to an improved process for preparing symmetrical and asymmetrical trans-thiazineindigo pigments.
Thiazineindigo pigments are an industrially important class of organic pigments. They are preparable by reaction of carbo- or heterocyclic o-aminomercapto compounds with maleic acid derivatives in polar aprotic solvents (DE-A-2 151 723, DE-A-2 536 120). An improved process is described in WO 98/32 800 A1. However, existing processes are in need of improvement with regard to yield and product quality.
It is an object of the present invention to provide a process for preparing trans-thiazineindigo pigments which provides higher yields, purer and more brilliant products and also ecological and economic advantages compared with existing processes.
This object is surprisingly achieved by a simple stage reaction by maintaining certain, hereinbelow defined temperature and pressure conditions.
The present invention accordingly provide a process for preparing symmetrical or asymmetrical trans-thiazineindigo pigments of the formula (1) 
where
R1 and R2 are identical or different and each represents a radical required to complete a substituted or unsubstituted aromatic or aliphatic carbocyclic or heterocyclic ring system, and
R3 is hydrogen, C1-C12-alkyl or phenyl,
which comprises
a) condensing, in an aqueous medium, a compound of the formula (2) with a compound of the formula (3a) or (3b) or a compound of the formula (4) with a compound of the formula (3a) and/or (3b) 
where
R1, R2 and R3 are each as defined above,
R4 is hydrogen or a metal ion selected from the group consisting of Na+, K+ and Zn2+/2,
R5 is a leaving group, and
Hal is Cl or Br;
by maintaining the reaction mixture at a temperature between 30 and 65xc2x0 C. for at least 1 hour and then
b) maintaining the reaction mixture at a temperature between 100 and 180xc2x0 C. and a pressure between 1.5 and 50 bar, preferably between 2 and 20 bar, for at least 1 hour in the presence of an amide.
In symmetrical trans-thiazineindigo pigments R1 and R2 are the same.
In asymmetrical trans-thiazineindigo pigments, R1 and R2 are different.
Symmetrical trans-thiazineindigo pigments are obtained when the compound of the formula (2) is reacted with the compound of the formula (3a), the preferred molar ratio for (2):(3a) being about 1:1 to 1:5, especially 1:1 to 1:1.1.
Symmetrical trans-thiazineindigo pigments are also formed when the compound of the formula (4) is reacted with approximately twice the molar amount of the compound of the formula (3a) or (3b), preferably in a molar ratio of (4):(3a) or (3b) of 1:2 to 1:5, especially 1:2 to 1:2.2.
Asymmetrical trans-thiazineindigo pigments are formed on reacting the compound of the formula (2) with the compound of the formula (3b), the preferred molar ratio for (2):(3b) being about 1:1 to 1:5, especially 1:1 to 1:1.1.
Asymmetrical trans-thiazineindigo pigments are also obtained on reacting the compound of the formula (4) with an approximately equimolar amount of the compound (3a) and an approximately equimolar amount of the compound (3b), preferably in a molar ratio of (4):(3a+3b) of 1:2 to 1:3.
By using a mixture of the compounds (3a) and (3b) it is possible to obtain mixtures of asymmetrical products and the two symmetrical products, especially when the molar ratio of (3a):(3b) is not equimolar.
Instead of the compounds of the formulae (3a) and (3b) it is also possible to use the corresponding aminothiazoles of the formulae (3c) and (3d) 
which are usually commercially available. Alkaline ring cleavage, for example in the presence of aqueous potassium hydroxide or sodium hydroxide solution and at temperatures between 100 and 200xc2x0 C., preferably between 110 and 150xc2x0 C., provides the o-aminomercapto compounds of the formulae (3a) and (3b), which are reacted as described above, if appropriate after acidification.
It is an important process engineering advantage that the process of the invention can be carried out as a simple stage reaction. The ring closure reaction, i.e., the condensation to the cis-thiazineindigo, takes place at temperatures between 30 and 65xc2x0 C. in step a). Without the cis product being isolated, the reaction mixture is then heated under pressure in the presence of an amide (step b), and the isomerization to the desired trans product and the pigmentary conditioning take place at one and the same time. The amide can be present in the reaction mixture from the start and/or added prior to the heating as per b). The amount of amide is advantageously between 5 and 70% by weight, especially between 20 and 40% by weight, based on the total weight of the reaction mixture in step b). Useful amides are preferably cyclic carboxamides and sulfonamides, especially N-methylpyrrolidone.
The aqueous medium contains water, if appropriate an acid or a base and optionally a water-soluble solvent, for example an alcohol. Useful acids include for example carboxylic acids, for example acetic acid, inorganic acids, for example sulfuric acid, hydrochloric acid or phosphoric acid. It is also possible for salts of these acids, especially buffers, to be present. Useful bases include for example sodium hydroxide and potassium hydroxide or organic bases, such as triethylamine.
The acids are advantageously present in the aqueous medium in such an amount that the pH is between 0 and 3, preferably between 1 and 2, especially between 1 and 1.5, in steps a) and b). The amount of acid is preferably 10 to 40% by weight, especially 10 to 20% by weight, based on the total weight of the reaction mixture.
It has been found that the formation of coloristically unwelcome monocondensation products can be substantially suppressed by maintaining a certain temperature profile. The reaction mixture is maintained at a temperature between 30 and 65xc2x0 C., preferably between 35 and 50xc2x0 C., for at least 1 hour, preferably 1 to 6 hours, especially 2 to 4 hours.
To raise the space-time yield, it can be advantageous to distill some or all of the water out of the reaction medium after the condensation has taken place.
For isomerization and conditioning, the reaction mixture is heated in the presence of the amide and maintained at 100 to 180xc2x0 C., preferably 110 to 160xc2x0 C., especially 130 to 150xc2x0 C., for at least 1 hour, preferably 1 to 8 hours, especially 2 to 5 hours. In a closed system, the pressure will automatically rise to 1.5 to 50 bar, preferably 2 to 50 bar, especially 5 to 10 bar.
The trans-thiazineindigo pigment formed is isolated by customary methods, for example distillative removal of the liquid medium or filtration of the pigment, washing and drying.
In a preferred embodiment of the process according to the invention, the reaction is effected in the presence of additives which inhibit the crystallization, for example quinacridonecarboxamides, quinacridonesulfonamides, thiazineindigocarboxamides or thiazineindigosulfonamides, for example those of the following formula:
P-Xm,
where
P is a residue of a linear unsubstituted quinacridone or of a linear 2,9-dimethylquinacridone or of a thiazineindigo radical,
m is from 1 to 4, and
X is a group of the formula xe2x80x94CONR8R9 or xe2x80x94SO2NR8R9, where R8 and R9 are independently H, C1-C18-alkyl, phenyl, naphthyl, C5-C10-cycloalkyl or 
Furthermore, the presence of dispersing aids, for example a (copyright)Sandopan, or wax can be advantageous.
In a further preferred embodiment of the process according to the invention, the reaction mixture is subjected to grinding, for example by means of a colloid mill, during the condensation and/or during the isomerization.
Preferred compounds prepared by the present process are those of the formula (1) where R1 and R2 are each a radical of the formula (a) to (i) 
where
Y is a halogen, preferably chlorine or bromine, or a pseudohalogen, preferably CN,
R is branched or linear C1-C8-alkyl, preferably C1-C4-alkyl, especially methyl or ethyl,
a is 1, 2, 3 or 4, and
b is 1, 2, 3 or 4.
R3 is preferably hydrogen, C1-C6-alkyl or phenyl, especially hydrogen, methyl.
In the starting compound of the formula (2), R5 is preferably OH, Cl, Br, C1-C5-alkoxy.
The process of the invention provides particularly pure and brilliant trans-thiazineindigo pigments.
More particularly, the fraction of coloristically unwelcome monocondensation product is usually below 5% by weight, as measured by HPLC, whereas it is frequently formed in an amount of 10 to 15% by weight in hitherto customary processes.
Moreover, the simple stage reaction of the invention has significant ecological and economic advantages, since there is no need to isolate the cis product formed in the course of the condensation and inconvenient filtration, washing and drying steps can accordingly be eliminated. It is surprising that the presence of amides, especially NMP, will cause the cis product to isomerize to the trans product in high yields while at the same time undergoing pigmentary conditioning.
It may be pointed out in this connection that direct synthesis of the trans product, for example from the corresponding fumaric acid derivative instead of the maleic acid derivative of the formula (4), is unsuitable for industrial purposes, since the fumaric acid derivative is very costly and not commercially available.